Why is my HTTP PUT request failing in Flow?

Shopify Plus
1

Im trying to make a PUT request from Flow to the PriceRule resource. The objective is to add a customer to a discount. I have tried the request in Postman and it works perfectly.

But when I input the request into Flow and the action gets triggered, I get this error:

400 Bad Request {“errors”:{price_rule”:”Required parameter missing or invalid”}}

This is what I’m inputing into Flow:

HTTP Method: PUT

URL: https://{API key}:{API password}@{my_store}.myshopify.com/admin/api/2019-07/price_rules/{price_rule_id}.json

Headers: 
    Key: X-Shopify-Access-Token
    Value: {API password}

Body: 
{
    "price_rule": {
        "customer_selection": "prerequisite",
        "prerequisite_customer_ids": [ 5678765*** ] 
    }
}

What parameter am I missing and where can I find examples or more detailed docs for the future?